This invention relates to a process for producing a dithionite.
In general, in a process for producing an anhydrous dithionite crystal by reacting formic acid or a formate, an alkaline compound and sulfur dioxide in the presence of water and an organic solvent, a dithionite crystal is crystallized in the reaction solution during the reaction. After the reaction completed, the resulting dithionite crystal can be separated from the mother liquor to recover it. However, if the dithionite crystal is dried as it is, purity of the product is lower because impurities contained in the mother liquor remain on the crystal, and water contained in the mother liquor delays the drying and accelerates decomposition of the resulting dithionite. When the decomposing speed of the dithionite is great, the dithionite crystal is likely to burn fire. This is very dangerous.
In order to overcome such shortcomings, it is indispensable to wash the dithionite crystal with an organic solvent, such as methanol or ethanol, thereby removing from the crystal water and impurities-containing mother liquor adhered to the crystal. The mother liquor is transferred to the washings by the washing step. The resulting washings contain thiosulfates, bisulfites, formates and water contained in the mother liquor. Particularly, when the washings containing thiosulfates are used as a solvent for synthesis of a dithionite without distillation, formation of a dithionite is not only prevented, but also decomposition of the dithionite is accelerated. Therefore, in the prior art the washings alone or combination of the washings and the mother liquor are distilled to recover a solvent from the washings or the combination.
For example Japanese Patent Publication (for opposition) No. 38556/1973 discloses recovery of an alcohol from the washings and reuse of the alcohol. However distillation of an organic solvent from the washings requires a great amount of energy. It is necessary from the view point of energy saving to solve such problem.